dandelany / react-dnd

Flexible HTML5 drag-and-drop mixin for React with full DOM control

react-dnd

HTML5 drag-and-drop mixin for React with full DOM control.

Note: both API and docs are in a bit of a flux now (haha). I'll try to get them stable and more easily understandable during the course of the following weeks, but meanwhile I appreciate feedback and corrections to the docs.

Prior Work

Check these first and see if they fit your use case.

• react-draggable by Matt Zabriskie
• react-sortable by Daniel Stocks
• react-dropzone by Param Aggarwal

If they don't, read on.

Examples

After cloning the project, run:

npm install
npm run
open http://localhost:8080/

This will run the included examples.
Or you can view them in browser (and see their source code).

Installation

npm install --save react-dnd

Dependencies: Flux and a couple of functions from lodash-node;
Peer Dependencies: React >= 0.11.0.

Note: I'm using ES6 features in this library, so you may want to enable Harmony transforms in JSX build step. This library has to be used with a bundler such as Webpack (or Browserify).

Rationale

Existing drag-and-drop libraries didn't fit my use case so I wrote my own. It's similar to the code we've been running for about a year on Stampsy.com, but rewritten to take advantage of React and Flux.

Key requirements:

• Emit zero DOM or CSS of its own, leaving it to the consuming components;
• Impose as little structure as possible on consuming components;
• Use HTML5 drag and drop as primary backend but make it possible to add different backends in the future;
• Like original HTML5 API, emphasize dragging data and not just “draggable views”;
• Support dropping files;
• Hide HTML5 API quirks from the consuming code;
• Different components may be “drag sources” or “drop targets” for different kinds of data;
• Allow one component to contain several drag sources and drop targets when needed;
• Make it easy for drop targets to change their appearance if compatible data is being dragged or hovered;
• Make it easy to use images for drag thumbnails instead of element screenshots, circumventing browser quirks.

Hopefully the resulting API reflects that.

Snippets

Simple Drag Source

First, declare types of data that can be dragged.

These are used to check “compatibility” of drag sources and drop targets:

// ItemTypes.js
module.exports = {
  BLOCK: 'block',
  IMAGE: 'image'
};

(If you don't have multiple data types, this libary may not be for you.)

Then, let's make a very simple draggable component that, when dragged, represents IMAGE:

var { DragDropMixin } = require('react-dnd'),
    ItemTypes = require('./ItemTypes');

var Image = React.createClass({
  mixins: [DragDropMixin],
  
  configureDragDrop(registerType) {

    // Specify all supported types by calling registerType(type, { dragSource?, dropTarget? })
    registerType(ItemTypes.IMAGE, {

      // dragSource, when specified, is { beginDrag(), canDrag()?, endDrag(dropEffect)? }
      dragSource: {

        // beginDrag should return { item, dragAnchors?, dragPreview?, dragEffect? }
        beginDrag() {
          return {
            item: this.props.image
          };
        }
      }
    });
  },
  
  render() {

    // {...this.dragSourceFor(ItemTypes.IMAGE)} will expand into
    // { draggable: true, onDragStart: (handled by mixin), onDragEnd: (handled by mixin) }.

    return (
      <img src={this.props.image.url}
           {...this.dragSourceFor(ItemTypes.IMAGE)} />
    );
  }
);

By specifying configureDragDrop, we tell DragDropMixin the drag-drop behavior of this component. Both draggable and droppable components use the same mixin.

Inside configureDragDrop, we need to call registerType for each of our custom ItemTypes that component supports. For example, there might be several representations of images in your app, and each would provide a dragSource for ItemTypes.IMAGE.

A dragSource is just an object specifying how the drag source works. You must implement beginDrag to return item that represents the data you're dragging and, optionally, a few options that adjust the dragging UI. You can optionally canDrag to forbid dragging, or endDrag(dropEffect) to execute some logic when the drop has (or has not) occured. And you can share this logic between components by letting a shared mixins generate dragSource for them.

Finally, you must use {...this.dragSourceFor(itemType)} on some (one or more) elements in render to attach drag handlers. This means you can have several “drag handles” in one element, and they may even correspond to different item types. (If you're not familiar with JSX Spread Attributes syntax, check it out).

Simple Drop Target

Let's say we want ImageBlock to be a drop target for IMAGEs. It's pretty much the same, except that we need to give registerType a dropTarget implementation:

var { DragDropMixin } = require('react-dnd'),
    ItemTypes = require('./ItemTypes');

var ImageBlock = React.createClass({
  mixins: [DragDropMixin],
  
  configureDragDrop(registerType) {

    registerType(ItemTypes.IMAGE, {

      // dropTarget, when specified, is { acceptDrop(item)?, canDrop(item)? enter(item)?, over(item)?, leave(item)? }
      dropTarget: {
        acceptDrop(image) {
          // Do something with image! for example,
          DocumentActionCreators.setImage(this.props.blockId, image);
        }
      }
    });
  },
  
  render() {

    // {...this.dropTargetFor(ItemTypes.IMAGE)} will expand into
    // { onDragEnter: (handled by mixin), onDragOver: (handled by mixin), onDragLeave: (handled by mixin), onDrop: (handled by mixin) }.

    return (
      <div {...this.dropTargetFor(ItemTypes.IMAGE)}>
        {this.props.image &&
          <img src={this.props.image.url} />
        }
      </div>
    );
  }
);

Drag Source + Drop Target In One Component

Say we now want the user to be able to drag out an image out of ImageBlock. We just need to add appropriate dragSource to it and a few handlers:

var { DragDropMixin } = require('react-dnd'),
    ItemTypes = require('./ItemTypes');

var ImageBlock = React.createClass({
  mixins: [DragDropMixin],
  
  configureDragDrop(registerType) {

    registerType(ItemTypes.IMAGE, {

      // Add a drag source that only works when ImageBlock has an image:
      dragSource: {
        canDrag() {
          return !!this.props.image;
        },
        
        beginDrag() {
          return {
            item: this.props.image
          };
        }
      },
  
      dropTarget: {
        acceptDrop(image) {
          DocumentActionCreators.setImage(this.props.blockId, image);
        }
      }
    });
  },
  
  render() {

    return (
      <div {...this.dropTargetFor(ItemTypes.IMAGE)}>

        {/* Add {...this.dragSourceFor} handlers to a nested node */}
        {this.props.image &&
          <img src={this.props.image.url}
               {...this.dragSourceFor(ItemTypes.IMAGE)} />
        }
      </div>
    );
  }
);

Dropping Files from the Hard Drive

The library provides one built-in item type: NativeDragItemTypes.FILE. You can't set up a drag source for it, but you can set up as many drop targets as you want with the same API as normal drop targets. The only way in which file drop target differs from a normal one is that item parameter will always be null in enter, over and leave. In drop, its files property will contain a JS array of filenames as given by browser.

var { DragDropMixin, NativeDragItemTypes } = require('react-dnd');

var ImageUploader = React.createClass({
  mixins: [DragDropMixin],
  
  configureDragDrop(registerType) {
    registerType(NativeDragItemTypes.FILE, {
      dropTarget: {
        acceptDrop(item) {
          // Do something with files
          console.log(item.files);
        }
      }
    });
  },
  
  render() {
    var fileDropState = this.getDropState(NativeDragItemTypes.FILE);

    return (
      <div {...this.dropTargetFor(NativeDragItemTypes.FILE)}>
        {fileDropState.isDragging && !fileDropState.isHovering &&
          <p>Drag file here</p>
        }
        {fileDropState.isHovering &&
          <p>Release to upload a file</p>
        }
      </div>
    );
  }
);

What Else Is Possible?

I have not covered everything but it's possible to use this API in a few more ways:

• Use getDragState(type) and getDropState(type) to learn if dragging is active and use it to toggle CSS classes or attributes;
• Specify dragPreview to be Image to use images as drag placeholders (use ImagePreloaderMixin to load them);
• Say, we want to make ImageBlocks reorderable. We only need them to implement dropTarget and dragSource for ItemTypes.BLOCK.
• Suppose we add other kinds of blocks. We can reuse their reordering logic by placing it in a mixin.
• dropTargetFor(...types) allows to specify several types at once, so one drop zone can catch many different types.
• When you need more fine-grained control, most methods are passed drag event that caused them as the last parameter.

API

require('react-dnd').DragDropMixin

configureDragDrop(registerType)

Gives you a chance to configure drag and drop on your component.
Components with DragDropMixin will have this method.

registerType(type, { dragSource?, dropTarget? })

Call this method to specify component behavior as drag source or drop target for given type. This method is passed as a parameter to configureDragDrop.

getDragState(type)

Returns { isDragging: bool } describing whether a particular type is being dragged from this component's drag source. You may want to call this method from render, e.g. to hide an element that is being dragged.

getDropState(type)

Returns { isDragging: bool, isHovering: bool } describing whether a particular type is being dragged or hovered, when it is compatible with this component's drop source. You may want to call this method from render, e.g. to highlight drop targets when they are comparible and when they are hovered.

dragSourceFor(type)

Returns props to be given to any DOM element you want to make a drag source. Intended to be used with JSX spread attribute syntax.

dropTargetFor(types...)

Returns props to be given to any DOM element you want to make a drop target. Intended to be used with JSX spread attribute syntax.

===================

Drag Source API

Implement to specify drag behavior of a component:

• beginDrag(e: SyntheticEvent) — return value must contain item: Object representing your data and may also contain dragPreview: (Image | HTMLElement)?, dragAnchors: { horizontal: HorizontalDragAnchors?, vertical: VerticalDragAnchors? }?, effectsAllowed: DropEffects[]?.

• canDrag(e: SyntheticEvent) — optionally decide whether to allow dragging.

• endDrag(effect: DropEffect?, e: SyntheticEvent) — optionally handle end of dragging operation. effect is falsy if item was dropped outside compatible drop targets, or if drop target returned null from getDropEffect().

===================

Drop Target API

To perform side effects in response to changing drag state, use these methods:

• enter(item: Object, e: SyntheticEvent)

• leave(item: Object, e: SyntheticEvent)

• over(item: Object, e: SyntheticEvent)

For example, you might use over for reordering items when they overlap. If you need to render different states when drop target is active or hovered, it is easier to use this.getDropState(type) in render method.

Implement these methods to specify drop behavior of a component:

• canDrop(item: Object): Boolean — optionally implement this method to reject some of the items.

• getDropEffect(effectsAllowed: DropEffect[]): DropEffect? — optionally implement this method to specify drop effect that will be used by some browser for cursor, and will be passed to drag source's endDrag. Returned drop effect must be one of the effectsAllowed specified by drag source or null. Default implementation returns effectsAllowed[0].

• acceptDrop(item: Object, e: SyntheticEvent, isHandled: bool, effect: DropEffect?) — optionally implement this method to perform some action when drop occurs. isHandled is true if some child drop target has already handled drop. effect is the drop effect you returned from getDropEffect, or if isHandled is true, drop effect of the child drop target that handled the drop.

===================

require('react-dnd').ImagePreloaderMixin

You can optionally specify images to be used as drag thumbnails. Browsers can't do this reliably until image is loaded, so ImagePreloaderMixin provides an API to do just that:

mixins: [DragDropMixin, ImagePreloaderMixin],

// This method should return array of image urls for preloading
getImageUrlsToPreload() {
  return ['some-img-url1', 'some-img-url2'];
},

// You can now use `this.hasPreloadedImage(url)` and `this.getPreloadedImage(url)` in your `dragSource`:
configureDragDrop(registerType) {
  registerType(ItemTypes.MY_ITEM, {
    dragSource: {
      canDrag() {
        return this.hasPreloadedImage('some-img-url1');
      },

      beginDrag() {
        return {
          item: ...,
          dragPreivew: this.getPreloadedImage('some-img-url1');
        };
      }
    }
  });
}

Above code will load the images after componentDidMount is executed, and cache them until component unmounted. In componentDidUpdate, mixin will check if getImageUrlsToPreload has changed, and load images again if needed.

Note that, for best results, you want to use this.getDragImageScale(). It will return correct scale to download for your images, considering browser differences in handling Retina screens (should either return 1 or window.devicePixelRatio). You can either use it with a custom server image resizer, or to choose between normal and @2x versions of predefined images.

require('react-dnd').NativeDragItemTypes

Provides a single constant, NativeDragItemTypes.FILE, that you can use as an item type for file drop targets.

require('react-dnd').DropEffects

Provides constants to be passed in effectsAllowed array from beginDrag() and returned from drop target's getDropEffect(). Correponds to singular native dataTransfer.dropEffect values.

Thanks

This library is a React port of an API, parts of which were originally written by Andrew Kuznetsov.